Not sure what killed her but, sorry for your loss! By the way, not sure what state you are in however here in Va. if you are registered with the state you are allowed to shoot and kill hawks if your chickens are your sole source of income regardless of the fact that they are federally protected. Vutures are protected also and I would not put this past them either. I have a school of them that fly over quite often and get a little close at times while admiring my flock of chickens maybe 30 feet up in the air and sometimes lower. I had spoken to Jeff Raumbaugh with Birds of Prey division at the extension office a while back regarding hawks and other pests and he is the one that had informed me that I can shoot them if all of the above were in place.
I had also read an article yesterday about how a large group of vultures, 50 or so, had hung around a womans house in Stauntan Va and the game warden had to kill a dozen or more for 2 or 3 consequtive Mondays and Tuesdays due to their killing one of their own and leaving it in this persons yard for a 9 year old to find. The article stated that vultures are vicious birds and if they kill calves and such while being born whos to say that they will never touch a chicken, duck, turkey or other barn yard animals when desperate for food! I wouldn't trust them either and my flock do not seem to fear them in which concerns me so since there are so many of these nasty birds.
By the way, I've had raccoon, as per game warden, or an owl gut a duck before and left the head in place as well as had accomplished a very clean job while feathers were left intact on the duck!